Currently, the main use of long ferrite cores is inner antennae in the fields of 10 KHz to 500 KHz. The effective permeability of a cylindrical core is proportional to the specific magnetic permeability of the material or μi times a form factor that is the L/D ratio, where L is the length and D is the diameter of the rod. This physical principle means that for the same ferromagnetic material, and antenna or inductor, has a larger inductance with product is longer and thinner, i.e. the L/D ratio is higher.
This principle led the designers to used ferrite cores with high L/D ratios that were wound with copper wire and then, protect the whole inductor by injecting it in a polymeric matrix or by casting it in a resin or, ultimately by providing an external protection in the form or a hard shell or box.
This solution obtained by common sinterization, and therefore being an intrinsically fragile solution, has been so far used in LF emitter antennas in Keyless entry systems for automotive as well as in induction soldering cannons and RF rod antenna for applications like atomic clock receivers among others.
The Young module (indicator of the elasticity of the ferrite) is very low, it means that ferrites are rigid and behave like glass or ceramic so they have fundamentally no deformation before cracking and braking.
A crack in a ferrite inside an antenna or inductor produces a high reluctance magnetic path of the field thus reducing the effective permeability and dropping the inductance, that if the application is a resonant tank for an antenna, leads to a higher self-resonant frequency of the tank that makes the circuit operate out of specifications or even do not operate at all as the energy transmitted to or by a not tuned tank can be too low to let the circuit operate as a signal transceiver.
To solve the above problems stacking foils of metallic soft magnetic materials have been used in this technical field These materials can be of several crystalline structures, including nano crystalline or amorphous alloys of Fe and other combinations of atomic Ni, Co, Cr or Mo or its multiple oxides. These solutions, known as laminations stacks or simply stacks are known for decades and have been massively used in electric 50 Hz and 60 Hz transformers among other applications. Metallic lamellae or bands in the form of stacks usually solve the problem of fragility but nevertheless, as they exhibit low ohmic resistivity, they need to be isolated from each other by isolating foils or layers of polymers, enamel, varnishes, and papers. A bendable antenna core is disclosed in US2006022886A1 and US2009265916A1 discloses an antenna core comprising a flexible stack of a plurality of oblong soft-magnetic strips consisting of an amorphous or nanocrystalline alloy. WO2012101034A1 discloses an antenna core being embodied in strip-shaped fashion and consisting of a plurality of metal layers composed of a nanocrystalline or amorphous, soft-magnetic metal alloy. In this case, the strip-shaped antenna core has a structure which extends along the transverse direction of the strip-shaped antenna core and which is elevated in a direction perpendicular to the plane of the strip-shaped antenna core
EP0554581B1 discloses a flexible magnetic core and a method for producing the same, the latter comprising mixing in a vacuum a powder of small particles of soft magnetic material with a synthetic resin, and then curing of the resin in the form of a block applying during said curing a strong magnetic field thereto such that the particles form mutually insulated, longitudinally stretched, persistent chains parallel to the applied magnetic field. The mixing is performed in a vacuum
The chains generated with such a method are provided by discrete powder particles with irregular cross-sections, the powder small particles having high probabilities of aggregating to each other between different chains unless very strong disaggregating agents and strong dispersant agents are used, as the mixture is in a very low viscosity form, this imposing severe complexity and cost. If chains of particles contact each other, there appear losses of charges (Foucault losses). And EP0554581B1 only provides as example of said soft magnetic material soft iron which is not suitable to operate to frequencies over 1 KHz.
U.S. Pat. No. 5,638,080A discloses an HF antenna comprising a sheet-like, flexible multipart magnet core manufactured of ferromagnetic material with an antenna winding which is made up of a plurality of turns and surrounds the magnet core. The turns of the antenna winding are formed by printed wiring arranged on a flexible film surrounding the magnet core. The magnet core is made up using individual plates, for example of insulated ferromagnetic material or amorphous alloy, which are embedded in a base material, also called carrier material, taking the form of a chain i.e. rigid elements (plates) connected by a flexible element (base material). Therefore the plates are not flexible and the flexibility of said magnetic core can be achieved only by the base material deformation on the direction perpendicular to said plates.
U.S. Pat. No. 5,159,347A discloses microscopic strips of high permeability magnetic conductor which are arrayed in a proximate relation to an electrical conductor to form paths for magnetic circuits about the electrical conductor. The strips may take various forms including filaments, such as one hundred micron microwire, and deposited submicron-sized layers of amorphous magnetic material. Moreover, the magnetic circuits may be closed with the strips forming a plurality of bands around the electrical conductor, and the magnetic circuits may be open, such as with the strips arrayed linearly adjacent to the electrical conductor. The magnetic circuits have numerous applications, including a variety of antennas, inductive wires, antenna ground planes, inductive surfaces, magnetic sensors, and direction finding arrays.